/* Copyright 2012 The Netty Project
 * 
 * The Netty Project licenses this file to you under the Apache License, version 2.0 (the "License"); you may not use
 * this file except in compliance with the License. You may obtain a copy of the License at:
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations under the License. */
package io.netty.example.factorial;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;
import io.netty.handler.codec.CorruptedFrameException;

import java.math.BigInteger;
import java.util.List;

/**
 * Decodes the binary representation of a {@link BigInteger} prepended with a magic number ('F' or 0x46) and a 32-bit
 * integer length prefix into a {@link BigInteger} instance. For example, { 'F', 0, 0, 0, 1, 42 } will be decoded into
 * new BigInteger("42").
 */
public class BigIntegerDecoder extends ByteToMessageDecoder {

	@Override
	protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
		// Wait until the length prefix is available.
		if (in.readableBytes() < 5) {
			return;
		}

		in.markReaderIndex();

		// Check the magic number.
		int magicNumber = in.readUnsignedByte();
		if (magicNumber != 'F') {
			in.resetReaderIndex();
			throw new CorruptedFrameException("Invalid magic number: " + magicNumber);
		}

		// Wait until the whole data is available.
		int dataLength = in.readInt();
		if (in.readableBytes() < dataLength) {
			in.resetReaderIndex();
			return;
		}

		// Convert the received data into a new BigInteger.
		byte[] decoded = new byte[dataLength];
		in.readBytes(decoded);

		out.add(new BigInteger(decoded));
	}
}
